This invention relates generally to arc welding processes, and more particularly to a method and an apparatus for monitoring an arc welding process in real-time to measure weld penetration.
In order to obtain adequate strength and integrity in a welded joint, it is necessary that proper weld penetration be achieved so that upon resolidification of the molten metal of the weld pool good adhesion between the metal parts forming the joint is achieved. Too little penetration results in incomplete adhesion between the metal parts throughout the thickness of the joint, and too much penetration, or burn-through, may produce undue weakening of the base metal of the parts surrounding the weld joint.
The integrity and quality of an arc welded joint is determined largely by the weld pool geometry during the welding process, along with the metallurgical properties of the weld metal and the heat effect on the base metal. Because variations in local metal thickness and composition, joint width, heat sinking and grounding geometry may cause variations in bead width, penetration, and the resulting seam geometry, there exists a need, particularly in automatic welding processes, for non-contacting sensors which are capable of reliably measuring weld penetration in real-time from the torch side of the weld. Various techniques have been proposed for accomplishing this. However, these techniques suffer from one or more disadvantages, or otherwise have proved to be unsatisfactory.
One such technique involves the measurement of the planar surface geometric characteristics of weld pools during the arc welding process by optically sensing the weld pool, as with a television camera, to measure the pool width and area. However, with certain materials, notably nickel-based superalloys, small variations in minor constituent elements can cause wide variations in weld puddle (pool) depth, i.e., penetration, even though the torch-side appearance of the puddle remains unchanged. Penetration may, of course, be detected if there is optical access to the underside of the workpiece, and this technique has been employed in some situations, but it is usually impractical in most industrial welding situations.
It is known that it is possible to determine the cross-sectional profile of a weld puddle if the puddle surface geometry, primarily its width, and natural oscillation frequency of the molten metal are known. It is further known that the natural oscillation frequency of the puddle drops when full penetration of the workpiece is achieved. Accordingly, another approach which has been used is to excite the weld pool to oscillation, as by applying a current pulse to the electrode of the arc welding torch, and by monitoring the arc voltage and analyzing the voltage variations to derive the oscillation frequencies of the weld pool. Arc voltage, which is influenced by gas composition, electrode wear, puddle surface impurities, and a host of other effects besides arc length is not an ideal parameter for sensing puddle oscillations. Moreover, in moving welds, the electrode is displaced from the geometric centroid of the weld pool where puddle oscillation amplitude would be maximized at full penetration, thereby rendering it harder to detect the oscillations by monitoring the arc voltage.
There exists a need for a method and an apparatus for reliably and easily measuring weld penetration in an arc welding process which avoid the foregoing and other disadvantages, and it is to this end that the present invention is directed.